The Nārada Purāṇa while listing the contents of Viṣṇu Purāṇa does include the Viṣṇudharmottara, however, it's listed towards the end of the chapter.
The Viṣṇu Purāṇa: The Table of Contents
Śrī Brahmā said:
1. O dear child, listen. I shall narrate the great Purāṇa of Viṣṇu. It contains twenty-three thousand verses. It is destructive of all sins.
2. In the first section, six divisions have been made by the descendant of Śakti i.e. Parāśara. In the first division the introduction to the Purāṇa is made to Maitreya.
16-17a. The story of Kali is mentioned, the four types of dissolution. The Knowledge of Brahman reviewed by Keśidhvaja (for the benefit) of Khaṇḍikya. So the sixth division is recounted.
17b-18. Hereafter, the perpetual virtues called Viṣṇudharmottaras are mentioned by Sūta on being respectfully asked by Śaunaka and others. Many holy stories, holy rites, observances, and restraints were also described by him.
19-20. Dharma-śāstra (Code of Laws), Arthaśāstra (Political economy), Vedānta (Metaphysics), Jyotiṣa (Astronomy and Astrology), the narration of the races, hymns according to the context; and dynasties, Manvantaras (or Manus); different vidyās (lores) that are beneficial to the worlds are described. This is verily the Viṣṇu Purāṇa which is a succinct collection of all the topics of sacred scriptures.
And as this footnote by the translator (G. V. Tagare) suggests, when other Mahāpurāṇas say the Viṣṇu Purāṇa contains 23,000 verses, the verses from Viṣṇudharmottara are also taken into account:
Though the NP. and some other Purāṇas such as the Vāyu, Matsya, Bhāgavata and Brahma-Vaivarta state that VP consists of 23000 verses, the printed editions give only 6000 verses.
The Gita Press edition has 6373 verses. It appears that the NP. in verses 17b-18, regards VP and Viṣṇudharmottara Purāṇa as one book while Śaṅkara (800 A.D.) regards them as two separate works (PJ VIII 94-96) showing thereby that the present NP. was settled before 800 A.D.